Titanium coated article



3,045,333 TlTANlIUM CGATED ARTICLE Walter. L. Finlay, Beaver, Pa., assignor to Rem-Cm Titanium, Inc, Midland, lPa., a corporation of Pennsylvania No Drawing. Filed Oct. 18, 1951, Ser. No. 251,979

, 4- Claims. (Cl. 29-194) This invention relates to titanium and titanium base alloys, and contemplates materially enhancing the utility of these materials, particularly for high temperature service, by the use of coatings of other metals which bond themselves to the base metal by penetrating and "alloying or combining therewith.

As structural materials, titanium and its alloys ofier a unique combination of high tensile strength and low density, but their utilization for high temperature service has been handicapped by their susceptibility to corrosion by atmospheric gases under some service conditions. At elevated temperatures both oxygen and nitrogen migrate from the surface into the interior of bodies of titanium and titanium base alloys, embrittling the whole structure. Moreover, titanium cannot be soldered, and is diiiicult to draw, due to its tendency to gall or adhere strongly to drawing tools and dies".

The present invention comprises the discovery that coatings of aluminum, tin, copper, lead and other metals can be app-lied to a titanium base in such a way as to combine or alloy with the surface metal, thereby forming a permanently bonded protective and non-galling coating. A copper coating can be secured by dipping for from 5 to 35 seconds in fused cuprous chloride or an admixture thereof with other chlorides at a temperature of 650 C. to 700 C. Such coatings can be soldered. A lead coating which enables wire and deep drawing can be's'ecured by immersing for a few minutes in a lead bath :at 870 C. to 925 C., the bath being cooled to 350 C. to 650 C.,

before the coated titanium is removed. An adherent tin coating can be secured by immersing carefully cleaned titanium in molten tin at 780-790 C. for one or tw minutes.

The preferred coating metal is aluminum. The titanium body to be coated is first thoroughly cleaned, as by grit blastinggrinding or the like, degreased with carbon tetcachloride or the like, and is then immersed in a bath of molten aluminum, preferably at a temperature between about 725 C. and 900 C. At somewhat higher temperatures, say 1'000 C., the molten aluminum attacks the titanium and forms a pasty sludge. A protective salt, such as potassium chloride, may be floated on the bath to minimize oxidation thereof.

The time of immersion varies with the bath temperature. At 900 C., good coatings have been secured with times as short as seconds and as long as 90 seconds, while at 725 C., the time range is from 1 to 8. minutes. A time of about 4 minutes is preferred. The coating secured by an immersion of 8 minutes is unnecessarily heavy, and some aluminum is lost by complete oxidation on subsequent exposure to high temperatures.

Specific examples of the practice of the invention are as follows:

Commercial titanium was rolled at a temperature of about 850 C to a thickness of .030". Specimens cut from this sheet were cleaned by sand blasting and mechanical grinding, finally polishing on 600-grit paper,

coated sheet were then heated in air for 24 hours at 105 0 C. The uncoated control samples were completely converted to titanium-dioxide, showing a weight gain of about 81 rug/sq. cm. The specimen which had been immersed for 1 minute showed a weight gain of about 18 mg./sq. cm., and was coated with a removable tan scale. The specimen which had been immersed for 2 minutes showed a weight gain somewhat over 20 mg./sq. cm., and was coated with a very adherent tan scale. The specimens immersed for 4 and 8 minutes showed weight gains of only about 9 rug/sq. cm., and were coated with very adherent gray scales. The surfaces of all specimens were substantially hardened but toward the center hardness decreased rapidly.

Other specimens, prepared and coated as above, were heated in air for 24-hours at 850 C., with even more satisfactory results. The uncoated control samples showed a weight gain of about 9 rug/sq. cm., while the aluminized specimens gained from .09 to 2.5 mg./sq. cm. The thickness of the aluminum oxide coating increased from about 2 mils to about 5 mils with increase in the immersion time. The oxide coating is relatively hard and is bonded to the titanium by a layer of about .5 mil thickness of an intermetallic compound having a hardness of about 250 Vickers-probably titanium-aluminum. This intermetallic layer shows plainly on micro-photographs of 100 magnification. The original hardness (210 Vickers) of the titanium within the intermetallic bonding layer remains substantially unchanged. Ductility is not materially altered, and aluminum clad titanium can be cold rolled to reduction without spalling or flaking.

Aluminum coatings can also be applied with beneficial results to titanium alloys which are embrittled by gas absorption at elevated temperatures. For example, an alloy of 10% manganese, 5% molybdenum, 5% chromium, balance titanium, as rolled and vacuum annealed, showed a bend ductility of zero. After air exposure at 300 C. for 60 hours, the aluminum-coated alloy still had a bend ductility of zero-T, while the bend ductility of the unv coated sample was 4T. After air exposure at 900 C.

for 5 minutes, the bend ductility of the uncoated specimen was 11T, while that of the coated specimen remained at zero-T.

and degreasing in carbon tetrachloride. The specimens The invention thus enables the use of titanium and its alloys at service temperatures substantially higher than has heretofore been deemed possible.

What is claimed is:

1. An article of titanium surface-coated with aluminum, the surface coating being bonded to the titanium base by an interfacial layer of the reaction product secured by immersing the clean titanium article in molten aluminum.

2. An article consisting predominantly of titanium,

surface coated with a layer of a metal selected from the group consisting of aluminum, tin, copper and lead, said layer being bonded to the titanium by an intermetallic compound of titanium and the selected metal.

3. A metal article protected from surface deterioration at elevated operating temperatures comprising a base consisting predominantly of titanium and a protective layer on said base of a metal selected from the group consisting of aluminum, tin, copper and lead, said layer being substantially continuous and bonded to said base.

4. A metallic article protected from surface deterioration at elevated operating temperatures comprising a base consisting predominantly of titanium and a protective layer on said base of aluminum, said layer being substantially continuous and bonded to said base.

(References on following page) Fatentecl July 24., 19oz References Cited in the file of this patent UNITED STATES PATENTS Thowless Jan. 5, 1915 Kirby Jan. 26, 1915 Austin Jan. 16, 1934 Pink June 1, 1937 Charlton June 1, 1937 Scheller Mar. 21, 1939 Whitfield et a1. Aug. 1, 1939 Scott Oct. 10, 1939 Ball June 17, 1941 Taylor May 5, 1942 4 Pink May 1, 1945 Whitfield et a1. Dec. 7, 1948 Thomas et a1. Mar. 22, 1949 Spence Aug. 23, 1949 Sears Dec. 13, 1949 Fink Feb. 14, 1950 Jenks Oct. 10, 1950 Apker June 12, 1951 Brennan Sept. 25, 1951 Alexander Apr. 6, 1954 Whitfield June 29, 1954 Pink July 6, 1954 Wainer Sept. 28, 1954 

1. AN ARTICLE OF TITANIUM SURFACE-COATED WITH ALUMINUM, THE SURFACE COATING BEING BONDED TO THE TITANIUM BASE BY AN INTERFACIAL LAYER OF THE REACTION PRODUCT SECURED BY IMMERSING THE CLEAN TITANIUM ARTICLE IN MOLTEN ALUMINUM. 